Thermocouples are electrical devices consisting of two different conductors forming an electrical junction. A temperature difference between the junction of the two different conductors and a region occupied by the two conductors remote from the junction results in a temperature-dependent voltage being generated as a result of the thermoelectric effect. This voltage can be used to determine temperature, and as such thermocouples are a widely-used type of temperature sensor.
The thermoelectric effect is the direct conversion of temperature gradients to electric voltage and vice versa. As such, a thermoelectric device, such as a thermocouple, generates voltage when there is a different temperature between the junction of the two different conductors and a region occupied by the two conductors away from the junction. The thermoelectric effect can be used to measure temperature (by measuring induced voltage caused by a temperature difference) or to change the temperature of objects (by applying a voltage).
Thermopiles are devices employing multiple thermocouples, usually connected in series, and can be used for measuring the intensity of incident radiation, typically visible or infrared light.
Prior art thermopiles are described in U.S. Pat. No. 4,111,717 and US published patent application no. US 2007/0034799 A1, the entire contents of each of which are incorporated herein by reference.
The basic structure of an infrared sensor 100 employing a thermopile is illustrated in FIG. 1. The sensor comprises: a baseplate 101 (e.g. silicon substrate with an insulator coating); a membrane portion 102 (represented by dashed line), where the base plate has been partially back-etched to leave a much thinner portion in the centre of the sensor; and the thermopile 103 itself comprising a plurality of thermocouple elements, each comprising two different conductors 104 and 105. The thermopile generates an output voltage Vout proportional to the temperature difference between the membrane portion (hot side, ‘H’) and the outer baseplate portion (cold side, ‘C’). The individual thermocouples are connected in series in a chain, such that going in a clockwise manner around the perimeter of the thermopile one encounters a conductor of type 104 connected (at the cold side, ‘C’) to a conductor of type 105 connected (at the hot side, ‘H’) to a conductor of type 104 and so forth around the thermopile.
In operation, incident radiation raises the temperature of the membrane portion 102 more, or more quickly, than the outer baseplate portion 101, owing to the lower thermal capacity of the membrane portion, which results in the hot side contacts of the thermopile increasing (or increasing more quickly) in temperature relative to the cold side contacts, thereby generating a measurable voltage Vout by the thermoelectric effect.
This voltage can be used to determine, or at least to provide a measure/indication of, the intensity of the incident radiation, in particular if certain quantities, such as the wavelength of the radiation, are known. This measure/indication of intensity will be more accurate if the thermopile itself is well-characterized. Prior art methods for thermopile characterization entail the inclusion of additional, permanent temperature sensitive elements and heaters in the infrared sensor. For example, US 2007/0034799 A1 discloses an infrared sensor which permanently includes a heater and thermo-sensitive resistors in an upper conductive layer of the device structure, which is shared with one or more layers of the thermopile itself. The thermo-sensitive resistors are placed in the cold and hot sides of the thermopile. In operation testing, a voltage is applied to the heater to increase the temperature of the membrane portion, and constant currents are applied to the thermo-sensitive resistors. This can be used to independently determine the temperature difference between the hot and cold sides of the thermopile. This can then be linked with a reading taken from the thermopile itself to test the operation of the sensor.